1. Field of the Invention
The present invention is directed to a method and apparatus for tracking the laser beam of an optical detection apparatus employed by a scanning probe microscope (SPM), and more particularly, to a tracking method and apparatus that enables top-down optical access to a probe of an atomic force microscope (AFM).
2. Description of Related Art
Several probe based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip of the probe to make a local measurement of one or more properties of a sample. More particularly, SPMs typically characterize the surfaces of small-scale sample features by monitoring the interaction between the sample and the tip of the associated probe. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
In addition, especially when measuring biological samples, the user often wants to view the sample, simultaneously or otherwise. With many types of AFMs, however, using an optical microscope in conjunction therewith poses a challenge given space limitations in and around the probe and sample. These challenges are described further herein in connection with the discussion of known AFM technology.
The atomic force microscope is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip extending from the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc. AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
The probe may be scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. supra; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs can be designed to operate in a variety of modes, including contact mode and oscillating flexural mode. In an oscillation “flexural mode” of operation the cantilever oscillates generally about a fixed end. One popular flexural mode of operation is the so-called TappingMode™ AFM operation (TappingMode™ is a trademark of the present assignee). In a TappingMode™ AFM, the tip is oscillated flexurally at or near a resonant frequency of the cantilever of the probe. When the tip is in intermittent or proximate contact with the sample surface, the oscillation amplitude is determined by tip/surface interactions. Typically, amplitude, phase or frequency of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction, most often using an optical deflection detection scheme. These feedback signals are then collected, stored, and used as data to characterize the sample.
A typical AFM system is shown in FIG. 1. An AFM 10 employing a probe device 12 including a base (not shown) and a probe 14, the probe 14 having a cantilever 15 supported by the base and a tip 17 on the free end of cantilever 15. An actuator or drive 16 drives probe 14 during operation. For Tapping Mode™ operation, drive 16 is an oscillating drive that drives probe 14 at or near the probe's resonant frequency. Commonly, an electronic signal is applied from an AC signal source 18 under control of an AFM controller 20 to drive 16, thus operating to oscillate probe 14, preferably at a selected free oscillation amplitude Ao. Notably, Ao can be varied over a broad range, e.g., from microns to nanometers, the latter being typically used for non-contact force measurements. As a practical matter, for low force interaction with the sample surface during imaging, Ao should be as small as possible, but large enough to prevent tip 17 from sticking to the sample surface 22 due to van der Waals and/or adhesive forces, for example. Probe 14 can also be actuated toward and away from sample 22 using a suitable actuator or scanner 24 controlled via feedback by computer/controller 20. Notably, the oscillating drive 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe. Moreover, though actuator 24 is shown coupled to the probe, actuator 24 may be employed to move sample 22 in three orthogonal directions as an X-Y-Z actuator.
In operation, as the probe 14 is brought into contact with sample 22, sample characteristics can be monitored by detecting changes in a characteristic of the deflection oscillation of probe 14. In particular, a deflection detection apparatus 26 employs a laser to direct a beam towards the backside of probe 14 which is then reflected towards a detector, such as a four-quadrant photodetector. As the beam translates across the detector, appropriate signals are transmitted to controller 20 which processes the signals to determine changes in the deflection/oscillation of probe 14. Commonly, controller 20 generates control signals to maintain a constant force between the tip and sample, typically to maintain a setpoint characteristic of the deflection/oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
During imaging, the control signals maintain the constant force most often by moving either the cantilever or the sample with respect to the other. By monitoring changes of the cantilever deflection/oscillation as a function of position over the surface and generating appropriate control signals, a map of the surface can be created. In particular, the sample surface is mapped by reading the control signals output by controller 20, which indicate relative motion of the cantilever and sample needed to keep the cantilever deflection oscillation constant.
Notably, some AFMs scan the sample (for example, in an XY plane) under a fixed probe defining a stylus or tip and a cantilever. There is, however, significant interest in AFMs that scan the tip over a fixed sample, including in Z, as shown in FIG. 1. This construction has a number of advantages, including the ability to image large samples that are not easily scanned. Nevertheless, some previous instruments that scan the tip suffer from compromises that do not allow them to take full advantage of the capabilities of AFMs that scan the sample. For example, one such scanned-tip AFM uses a fixed laser to measure cantilever deflection and thus has a maximum scan size set by the diameter of the laser beam at the cantilever. If the cantilever is scanned a distance larger than the beam size, it will move out from under the beam, and it will no longer be possible to detect the cantilever motion. Another scanned-tip design, however, mounts the laser and probe on the same scanning unit, so that they move together, and thus does not have this problem.
If the probe moves independently of the laser beam used to measure deflection, as in FIG. 1, then the beam needs to track probe movement. One solution for tracking the beam of an optical detection system employed in a scanned-tip AFM includes using a light source and a scanned optical assembly that guides light emitted from the light source onto a point of the cantilever. A moving light beam is thus created which will automatically track the movement of the cantilever during scanning. See U.S. Pat. Nos. 6,032,518, 5,714,682, 5,560,244, and 5,463,897, owned by Veeco Instruments Inc. Overall, the system of these patents is quite effective at tracking the laser beam with motion of the lever.
Though avoiding the scan size issue to an extent, such systems have the disadvantage that the scanner may have to carry the weight of a laser, electronic leads to the laser, a focusing lens, and/or any other mechanism for fine tuning the laser position on the cantilever. All of these devices can reduce the mechanical resonant frequency of the scanner and transmit vibrations to the cantilever. Even if the mechanism does not move the laser along with the scanner, the system still requires an optical assembly that is attached to the scanning mechanism adding weight to the scanner leading to these performance issues (See FIG. 4A of the '518 patent, for example). Also, in the case of a tube scanner, attachment of a laser to the interior of a scanner can make its exchange difficult, for example, in the case of laser failure. In addition, in these systems, the laser beam, the cantilever and the tip move with respect to a fixed position sensitive detector. So, when the cantilever is scanned over the surface of the sample, the reflected laser beam will move with respect to the fixed position sensitive detector even in the absence of any actual deflection of the cantilever, for example, due to limitations associated with piezoelectric scanners (e.g., motion not being perfectly linear, etc.).
Moreover, substantial difficulties are often encountered when attempting to properly align static correction optics such as those used in AFMs. There is always a limit to how precise the optics can be aligned, and there is an inherent variability in performance from scanner to scanner.
Moreover, as noted previously, though an optical microscope may be employed together with the tracking mechanisms of these patents, separate structures such as mirrors are required to bring in light to illuminate the probe/sample for the optical microscope. Due to inherent inefficiencies and substantially poorer performance, complete top-down optical access to the lever for viewing the same along with the sample is preferred. Herein, top-down optical access refers to disposing an objective and a condenser along an axis in line with an axis orthogonal to the probe and sample surface. Notably, however, in known systems, at least a portion of deflection detection apparatus or the scanner is mounted overhead of the probe and thus top-down optical access is compromised. Moving these components away from the region directly overhead of the probe, though creating challenges concerning tracking the laser beam as discussed further below, was therefore desired.
In sum, the field of atomic force microscopy has been in need of a system that tracks the laser along with deflection of the probe by preferably decoupling the tracking mechanism, including the light source and associated optical components, from the scanning mechanism used to scan the probe. Moreover, a system that is able to provide complete top-down optical access was preferred.